I believe that Nietzsche also wrote

"For one may doubt, first, whether there are any opposties at all, and seconly whether these popular valuaions and opposite values on which the metaphysicians put their seal, are not perhaps erely foreground estimates, only provisional perpsectives, perhaps even from some nook, perhaps from belo, frog perspectives, as it were , to borrow an expression painters use. For all the value that true, the truthful, the selfless may deserve, it would still be possilbe that a higher and more fundametnal value for life might have to be ascribed to deception, selfishness, and lust. It might even be possible that what constitutes the value of these good and revered things is precisely that they are insidiusly related, tied to, and involved with these wicked, seemingly opposite things - maybe even one with them in essence. Maybe!"

(Friedrich Nietzsche, Beyond Good & Evil, section 2, Walter Kaufmann translation)

In this forray into almost Eastern monism, Nietzsche is pointing out that Evil is really just a counterpoint to good. That the connection between equates that the two do not exist seperately. Furthermore, he goes on to talk about how Good is really based on a synthetic judgement a priori that is common among metaphysicians. The implication of this is would then by that niether good nor evil really exist, it's just our percetpion of things that brings them into existence.

Existentialism is going to deny the existence of pretty much anything that is not readily apperent in the physical world, but in Nietzsche's case there is the notable exeception of the Will to Power. Assuming that the Will to Power, though, is more or less equivalent to Freud's "unconcious mind" (while there are of course significant differences between the two concepts, the primary tenents are the same) that would mean that the Will to Power has a physical expression in the mind of individuals, just not in the concious mind, giving it the right to claim existence in an Existential world. Then, according to Nietzsche and indirectly according to Freud, all of our actions/thoughts are directed by the "Will to Power" that means all of our synthetic judgments a priori are also and expression of the Will to Power. This takes us back to "Good" being a synthetic judgement a priori and "Evil" being merely a logical counterpoint to "Good." This means that "Evil" is also a just a description of a synthetic judgment a priori and therefore has no external existence. So, yes, it's all just in our heads.

__________________
Yes, the little green fist up there is a symbol of Earth First! No, I'm not an eco-terrorist, nor do I agree with eco-terrorism...I just thought it looked cool.